Implemented in v12.5, released today. From the
change log:
New OmniSearch feature: If Quick Search is disabled for all fields in a table, the OmniSearch form (a single-field, search-all textbox) will appear instead, allowing you to search across all compatible fields. Some new, related, config parameters ($enable_omnisearch_feature, $omnisearch_fields_to_include, $include_date_fields_omnisearch, $include_file_fields_omnisearch) are now available.
When you install DaDaBIK (or install a table/view), now by default none of the fields are included in the quick search form, so the OmniSearch field is displayed by default.